Sustainable Drainage Systems Major Design Guidance

Infiltration:

On sites where infiltration is proposed to drain surface water, the applicant should provide a table for each soakaway including the following information: cover level, invert level, soakaway depth, area drained to soakaway, attenuation volume, rainfall design event, climate change allowance, factor of safety (in accordance with Table S3.1 from the National standards for sustainable drainage systems), depth to highest recorded ground water level and infiltration rate used.

Runoff rates:

Discharge rates should be limited to the lowest achievable rate for the designed storm event. The lowest achievable rate should be used where a sewerage undertaker states a maximum discharge rate in their developer enquiry response.

For brownfield sites the expectation is that the relaxation factor will only apply in exceptional circumstances.

For the majority of sites, where development is increasing the volume of runoff the allowable discharge rate will be limited to the 50% Annual Exceedance Probability (AEP) (1 in 2 year).

Exceedance events:

An exceedance flow route plan demonstrating no increase in surface water flood risk on or off site should be submitted. The plan must include proposed levels and flow directions. This information will be considered at the application stage, with the detailed design information required at the pre-commencement condition stage (if not supplied prior to this).